Fog reduction in photographic silver halide emulsions



3,161,520 EGG REDUCTION IN PHOTGGRAPHEC SILVER HALHDE EMULSHONS Emil B. Ranch, Port Dickinson, and Fritz Dersch, Binghampton, N.Y., assignors to General Aniline Film Corpnra Lon, New York, N.Y., a corporation of Deiaware a No Drawing. Filed Dec. 11, 1962, Ser. No. 243,745 15 Claims. (Cl. 96-409) This invention relates to photographic materials and to methods of preparing them. More particularly, this invention relates to light-sensitive silver halide emulsions containing as a stabilizing and antifogging agent a 3-hydroxymethylazolethione.

It is well known in the photographic art that lightsensitive emulsions, such as gelatino-sil-ver halide emulsions, have a tendency to fog. Fog is usually caused by a prolonged ripening of the emulsion, by prolonged storage especially at elevated temperatures and humidity, and by prolonged development. To overcome this undesirable property, it has been the practice in this art to add certain chemical compounds to the emulsions to increase their stability and to reduce their tendency to fog. However, the stabilizing and antifogging compounds heretofore usedin this art have the disadvantage that upon addition to the emulsion they cause a loss of speed and/ or contrast of the emulsion. a

It is, accordingly, an object of this invention to produce a light-sensitive emulsion which is fast, stable, has a reduced tendency to fog and has good contrast.

A further object of this invention resides in a lightsensitive emulsion which contains a compound which stabilizes the emulsion against fogging and at the same time does not materially reduce the speed and/or the contrast of the emulsion.

We have discovered that 3-hydroxymethylazolethiones when added to a light-sensitive silver halide emulsion, stabilize and inhibit the fogging of the emulsion.

The azolethiones, the use of which is contemplated herein, may be represented by the following formula:

time or bromine; a lower alkyl radical, e.g., methyl, ethyl, propyl, isopropyl, butyl, isobutyl, amyl; a carbon group;

arenas Patented Dec. 15, 1964 "Ice 3-hydroxymethyl-6-1nethylbenzoxazole-Z-thione N 0 Her- ('3 H2 0 H 3 -hydroxymethyl-5-methoxy-benzoxazole-Z-thione 3 -hydroxymethyl-5-chloro-benzoxazole-Z-thione 3 -hydroxymethyl-5-methyl-benzothiazole-2-thione 3 -hydroxymethyl-G-methyl-b enzothiazole-Z-thione 3 -hydroxyrnethyl-7 -methyl-b enzothiazole-Z-thione 3 -hydroxymethyl-6-tert.butyl-benzothiazole-Z-thione 3 -hydroxymethyl-5-chloro-benzothiazole-2-thione 3-hydroxymeth l-6-chloro benzothiazole-Z-thione 3 -hydr oxymethyl-S -bromo-benzothiazole-2-thione 3-hydroxymethyl-6-bromo-benzothiazole-2-thione 3 -hydroxymetl1yl-4,6-dimethyl-benzothiazole-Z-thione 3 -hydroxymethyl-4, 6-dimethyl-7-chloro-benZ0thiaZo1e-2- thione 3-l1ydroXymethyl-4-methyl-6-chloro-benzothiazole-2- thione 4, 6-dimethyl-5 ,7-dichloro-benzothiazole-2-thione 3 -hydroxymethyl-5-chloroh enzoxazole-2-thione 3 -hydroXymethyl-5 -methoxybenzoxazole-Z-thione 3 -hydroxymethyl-S-methylb enzoxazole-Z-thione These compounds are prepared by reacting a 2-mercapto-substituted azole of the benzoxazole and benzothiazole series with formaldehyde in the presence of a suitable solvent such as methanol, ethanol, isopropanol and the like. The preparation of these compounds is described in more detail in Berichte, vol. 90, page 2246, and Journal of the ChemicalSociety, 1948, page 1717.

As representatives of azoles used in the reaction, the following are mentioned:

Z-mercaptobenzothiazole 5-methyl-2-mercaptobenzothiazole 6-methyl-Z-mercaptobenzothiazole 7-tnethyl-2-mercaptobenzothiazole 6-tert.butyl-Z-mercaptobenzothiazole 5-methoxy-2-mercaptobenzothiazole a lower alkoxy group, e.g., methoxy, ethoxy, propoxy; I

and Y is an oxygen or sulfur atom.

The following specific compounds are examples of compounds belonging to the class of compounds having the general formula given above which we have found suitable forum as stabilizing and antifogging agents:

3-hydroXymethyl-benzoxazole-Z-thione i OHzQH 5-chloro-2-mercaptobenzothiazole 5-hromo-Z-mercaptobenzothiazole 6-chloro-2-mercaptobenzothiazole 6-bromo-Z-mercaptobenzothiazole 5 -carboxyl-Z-mercaptobenzothiazole 4,6-dimethyl-Z-mercaptobenzothiazole 4-methyl-6-chloro-2-mercaptobenzothiazole 5,6-dichloro-2-mercaptobenzothiazole 4-carboxy-6-chloro-Z-mercaptobenzothiazole 4,6-dimethyl-7-chloro-2-mercaptobenzothiazole 4,6-dimethyl-5,7-dichloro-2-mercaptobenzothiazole Z-mercaptobenzoxazole Z-mercap-to-S-chlorobenz0xazole 2-mercapto-5-rnethoxybenzoxazole Z-mercapto-S-methylbenzoirazole are added during-the ripening or sensitivity increasing stage of the emulsion making process. Such addition may be made before, during or after the addition of the soluble silver salt to the soluble halide in the presence of a suitable colloid, such as gelatin, polyvinyl alcohol, solubilized casein or albumin. When added as a coating final, the

antifogging agent of our invention is added to the emulsion just prior to coating it on a suitable support such as glass, paper, or film at a time when the emulsion has nearly attained its maximum sensitivity.

In some instances, it is advantageous to apply the antifogging and stabilizing compounds of our invention in a separate layer such as an undercoating layer or in an antiabrasion gelatin surface. Sometimes it is desirable to incorporate the compounds in one or all processing baths or in the preand post-baths.

When used as a ripening final, the antifoggants of our invention are preferably added to the emulsion in an amount ranging from 0.1 milligram to 20 milligrams per 0.6 mol of silver halide and when used as a coating final, it is preferably added in an amount ranging from 0.5 milligram to 50 milligrams per 0.6 mol of silver halide. The optimum amount to be added depends primarily on the type of emulsion and should be determined individually in each case.

The stabilizers and antifoggants of our invention may also be used in combination with known antifoggants and stabilizers. The antifoggants of our invention can also be used in combination with sensitizers such as sulfur, metal and reduction sensitizers as well as with speed-increasing agents and accelerators such as the reaction products of long-chain alcohols and ethylene oxide (see U.S.P. 1,970,578) and their derivatives and polyvinylpyrrolidone.

The novel antifoggants of our invention may be used with various types of photographic emulsions, such as non-sensitized orthochromatic, panchromatic and X-ray emulsions, paper emulsions and color emulsions.

The following specific examples are given as an illustration of the manner in Which the antifoggants of our invention can be used. It is to be understood, however, that these examples are given by way of illustration and not by way of limitation.

Example I A silver halide emulsion in gelatin containing 2 percent silver iodide and 98 percent silver bromide was prepared in a conventional manner and brought up to its maximum light sensitivity. It was then readied for coating, finals were added such as sensitizing dyes and hardening agents. A 0.1 percent solution of 3-hydroxymethylbenzothiazole-2-thione prepared according to the method described in the Journal of the Chemical Society, 1948, page 1717, was added in varying amounts to samples of the emulsion as an antifoggant and stabilizer. Each emulsion sample contained about 0.6 mol of silver halide. The so prepared emulsion samples were coated on a suitable cellulose ester base and dried. Samples of these film coatings were then exposed in a Type 1113 Sensitometer and developed in a developer of the following composition:

Grams Metol 1.5 Sodium sulfite (anhydrous) 45 Sodium bisulfite 1 Hydroquinone 6 Sodium carbonate (monohydrated) .8

Water to make 1 liter.

The developed samples were short-stopped, fixed,

Example [I The procedure followed was identical to Example I, except that in place of 3-hydroxymethylbenzothiazole-Z- thione the compound used was 3-hydroxymethylbenzoxazole-2-thione prepared according to the method described in Berichte, vol. 90, page 2250.

4', The results obtained were as follows:

Quantity of Compound Relative Fog at 12 Oven Fog at Used Speed Develop- 6 Development ment 10 Example III The procedure followed was identified to Example I, but in place of 3-hydroxymethyl-benzothiazole-2-thione, the compound used was 5-chloro-3-hydroxymethyl-benzothiazole-Z-thione which had been prepared in analogy to the method described in J.C.S., 1948, 1717, except that 5- chloro-2-mercaptobenzothiazole was used as the starting material in place of Z-mercaptobenzothiazole.

The results obtained were as follows:

Example I was repeated except that 6-methyl-3-hydroxymethyl-benzothiazole-Z-thione was used in place of 3-hydroxymethyl-benzothiazole-Z-thione. The results obtained were essentially the same as those reported in Example I.

The synthesis of this compound was carried out in analogy to the process described in J.C.S., 1948, 1717, except that 6-rnethyl-2-mercaptobenzothiazole was used as the starting material in place of fZ-mercaptobenzothiazole.

Our invention is not limited to the detailed description contained herein, but includes all modifications that fall within the scope of the appended claims.

This application is a continuation-in-part of our copending application Serial No. 765,339, filed October 6, 1958, now US. Patent 3,081,170.

What we claim is:

1. A light-sensitive silver halide emulsion containing as an antifoggant a compound selected from the group of those having the following general chemical formula:

CHzOH wherein R is selected from the group consisting of hydrogen, halogen, lower alkyl, lower alkoxy and carboxy; and Y is a member selected from the group consisting of oxygen and sulfur.

2. A light-sensitive silver halide emulsion as recited in claim 1, wherein the antifoggant is 3-hydroxymethylbenzothiazole-Z-thione.

3. A light-sensitive silver halide emulsion as recited in claim 1, wherein the antifoggant is 3-hydroxymethylbenzoxazole-Z-thione.

4. A light-sensitive silver halide emulsion as recited in claim 1, wherein the antifoggant is 5-chloro-3-hydroxymethyl-benzothiazole-Z-thione.

5. A light-sensitive silver halide emulsion as recited in claim 1, wherein the antifoggant is 6-methyl-3-hydroxymethyl-benzoxazole-Z-thione.

6. A light-sensitive silver halide emulsion as recited in claim 1, wherein the antifoggant is 5-chloro-3-hydroxymethyl-benzoxazole-2-thione.

7. A light-sensitive silver halide emulsion as recited in claim 1, wherein the antifoggant is present in the emulsion in the ratio of 0.1 to 50 milligrams per 0.6 mol of silver halide.

8. A light-sensitive silver halide emulsion as recited in claim 1, wherein the antifoggant is present in the emulsion in the ratio of 0.1 to 20 milligrams per 0.6 mol of silver halide.

9. A light-sensitive silver halide emulsion as recited in claim 1, wherein the antifoggant. is present in the emulsion in the ratio of 0.5 to 50 milligrams per 0.6 mol of silver halide.

10. A fight-sensitive silver halide emulsion. as recited in claim 1, wherein said emulsion is a panchromatic emulsion.

11. A light-sensitive silver halide emulsion as recited in claim 1, containing the reaction product of a long chain alcohol and ethylene oxide as an accelerator.

12. A light-sensitive photographic element comprising a base and a coating of geIat-ino-silver halide emulsion thereon, said emulsion containing as an antifogging and stabilizing agent, a compound selected from the group having the following formula:

0112011 wherein R is selected from the group consisting of hydrogen, halogen, lower alkyl, lower alkoxy; and Y is a member selected from the group consisting of oxygen and sulfur.

13. A light-sensitive structure comprising a base, a layer of light-sensitive silver halide emulsion thereon, and a separate layer adjacent said first-mentioned layer containing as an antifogging and stabilizing agent compound, a compound selected from the group having the following general chemical structure;

$52011 wherein R is selected from the group consisting of hydrogen, halogen, lower alkyl, lower alkoxy and carboxyalkoxy; and Y is a member selected from the group consisting of oxygen and sulfur.

14. A process of forming a photographic silver halide emulsion having a reduced tendency to fog which comprises forming the emulsion, ripening the emulsion and during said ripening adding thereto a compound selected from the group having the following general formula:

wherein R is selected from the group consisting of hydrogen, halogen, lower alkoxy and carboxy; and Y is a member selected from the group consisting of oxygen and sulfur.

References Cited in the file of this patent UNITED STATES PATENTS 2,870,015 Allen et a1 Jan. 20, 1959 3,026,201 Ranch et a1 Mar. 20, 1962 

1. A LIGHT-SENSITIVE SILVER HALIDE EMULSION CONTTAINING AS AN ANTIFOGGANT A COMPOUND SELECTED FROM THE GROUP OF THOSE HAVING THE FOLLOWING GENERAL CHEMICAL FORMULA: 